Nanoporous structures by reactive ion etching

ABSTRACT

A method for forming porous metal structures and the resulting structure may include forming a metal structure above a substrate. A masking layer may be formed above the metal structure, and then etched using a reactive ion etching process with a mask etchant and a metal etchant. Etching the masking layer may result in the formation of a plurality of pores in the metal structure. In some embodiments, the metal structure may include a first end region, a second end region, and an intermediate region. Before etching the masking layer, a protective layer may be formed above the first end region and the second end region, so that the plurality of pores is contained within the intermediate region. In some embodiments, the intermediate metal region may be a nanostructure such as a nanowire.

BACKGROUND

The present invention generally relates to nanoporous structures, and particularly to forming porous nanoscale metal structures by reactive ion etching.

Nanoscale (i.e., less than 100 μm) structures (“Nanostructures”) are increasingly having applications in numerous fields, such as metal-ion batteries, energy storage and chemical sensors. One such application is the use of palladium nanowires as hydrogen (H₂) sensors. Palladium (Pd) is capable of absorbing hydrogen to form a palladium hydride (PdH_(x), 0.0<x<˜0.67). Because the resistance of the palladium increases as hydrogen absorption increase, a Pd nanowire may be used to sense hydrogen by measuring the conductance across the Pd nanowire. A decrease in conductance may be correlated with an increase of hydrogen concentration in the region around the nanowire.

Nanostructures such as nanowires are particularly useful as chemical sensors due to their high surface-to-volume ratio. For example, because a palladium structure with a high surface-to-volume ratio can more easily absorb hydrogen, it may be more sensitive to environmental changes relative to non-nanoscale structures with lower surface-to-volume ratio. Because there is a correlation between surface-to-volume ratio and sensor performance, it follows that it may be advantageous to further increase the surface-to-volume ratio of palladium nanowires to further increase the sensitivity.

One manner of increasing the surface-to-volume ratio of a nanostructure is to increase it's porosity by forming a plurality of voids in the nanostructure. However, typical processes for forming porous nanostructures require bottom-up manufacturing approaches such as solution phase reactions or dispersion on a surface. Such processes may be difficult to control, scale up, or incorporate into typical fabrication flows. Therefore, a process of forming a porous nanostructure, for example a porous palladium nanowire, using typical semiconductor fabrication processes may be desirable.

BRIEF SUMMARY

According to one embodiment, a metal structure made be perforated, or made porous, by forming a masking layer above the metal structure, and then removing at least a portion of the masking layer using a reactive ion etching process with an etching gas containing a mask etchant and a metal etchant. While removing the portion of the masking layer, the metal etchant may sputter the metal structure, resulting in the formation of a plurality of pores in the metal structure. In some embodiments, the plurality of pores may have an average diameter of less than 10 nm.

In another embodiment, a porous metal structure may be formed by first forming a metal structure above a substrate. The metal structure may include a first metal end region, a second metal end region, and an intermediate metal region between the first metal end region and the second metal end region. In some embodiments, the intermediate metal region may include a nanostructure or nanostructures, such as a nanowire. A masking layer covering the metal structure may be formed above the metal structure. A protective layer may be formed above the masking layer covering the first metal end region and the second metal end region. The masking layer may then be etched away from above the intermediate metal region using a reactive ion etching process with a mask etchant and a metal etchant. The etching process may form a plurality of pores in the intermediate metal region.

Further embodiments may include a structure comprising a first metal end region, a second metal end region, and an intermediate metal region including a plurality of pores between the first metal end region and the second metal end region. The structure may be made of a metal such as palladium, gold, titanium, or platinum. The intermediate metal region may be a nanostructure such as a nanowire. The plurality of pores may have an average diameter of less than 10 nm, for example 5 nm.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description, given by way of example and not intended to limit the invention solely thereto, will best be appreciated in conjunction with the accompanying drawings, in which:

FIG. 1 is an isometric drawing depicting a semiconductor substrate, according to an embodiment of the present invention;

FIG. 2 is an isometric drawing depicting forming a metal layer above the semiconductor substrate, according to an embodiment of the present invention;

FIG. 3 is an isometric drawing depicting forming a metal structure from the metal layer, according to an embodiment of the present invention;

FIG. 4 is an isometric drawing depicting forming a first masking layer above the metal structure, according to an embodiment of the present invention;

FIG. 5 is an isometric drawing depicting forming a second masking layer above the first masking layer, according to an embodiment of the present invention;

FIG. 6 is an isometric drawing depicting etching the first masking layer and the metal structure, according to an embodiment of the present invention; and

FIG. 7 is an isometric drawing depicting the metal structure having a first end region, a second end region, and a porous middle region between the first end region and the second end region, according to an embodiment of the present invention.

Elements of the figures are not necessarily to scale and are not intended to portray specific parameters of the invention. For clarity and ease of illustration, scale of elements may be exaggerated. The detailed description should be consulted for accurate dimensions. The drawings are intended to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements.

DETAILED DESCRIPTION

Exemplary embodiments will now be described more fully herein with reference to the accompanying drawings, in which exemplary embodiments are shown. This disclosure may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of this disclosure to those skilled in the art. In the description, details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the presented embodiments.

Embodiments of the invention generally relate to methods of forming a porous metal structure using reactive ion etching. In the embodiments depicted in FIGS. 1-7, the metal structure may include a palladium nanowire connecting two metal pads on each end of the nanowire. Such a structure may be useful, among other things, as a hydrogen sensor, where the nanowire absorbs hydrogen and changes resistance as a result. By perforating the nanowire (i.e., by forming a plurality of pores in the nanowire), the surface area of the nanowire may increase, potentially resulting in an increased ability to absorb hydrogen and improved performance as a sensor. However, embodiments of the present invention are not limited to only nanowires, and the applications of a porous metal structure may extend beyond chemical sensors. After reading the specification, it will be apparent to one of ordinary skill in the art how embodiments further include a large number of porous metal structures, of both nanoscale and larger.

Referring to FIG. 1, a substrate 110 may be provided. The substrate 110 may be made of any material or materials capable of supporting the metal structure described below in conjunction with FIGS. 2-3 and capable of substantially withstanding the etching processes described below in conjunction with FIGS. 6-7. In some embodiments, the substrate 110 may be made of an insulating material such as glass. In other embodiments, the substrate 110 may be made of a semiconductor material such as silicon, germanium, silicon-germanium alloy, silicon carbide, silicon-germanium carbide alloy, and compound (e.g. III-V and II-VI) semiconductor materials. Non-limiting examples of compound semiconductor materials include gallium arsenide, indium arsenide, and indium phosphide.

Typically the substrate 110 may be about, but is not limited to, several hundred microns thick. For example, the substrate 110 may include a thickness ranging from approximately 0.5 mm to approximately 1.5 mm. In some embodiments, the substrate 110 may contain multiple layers of different materials and may contain various microelectronic or nanoscale structures, which have been omitted from the drawings for illustrative clarity.

FIGS. 2-3 depict forming a metal structure 210 (FIG. 3) above the substrate 110. In the depicted embodiment, the metal structure 210 is formed by depositing a metal layer 120 above the substrate 110 and etching the metal layer 120 using typical photolithographic processes. However, the metal structure 210 may also be formed by any other processes known in the art including, for example, a lift-off process, where a pattern may be first formed in a sacrificial layer and the pattern filled with metal before removing the sacrificial layer. A person of ordinary skill in the art will understand how to adapt the method described below to accommodate other methods of forming the metal structure 210.

Referring to FIG. 2, a metal layer 120 may be deposited above the substrate 110. The metal layer 120 may be made of a metal such as palladium, gold, titanium, or platinum. In other embodiments, the metal layer 120 may be made of a metal alloy, such as palladium-nickel or palladium-cerium. Depending on the deposition process used, the metal layer 120 may be as thin as a few angstroms. While there is no upper bound on the thickness of the metal layer 120, the process described below in conjunction with FIGS. 4-6 to perforate portions of the metal layer 120 may result only in pitting of the metal layer 120 rather than holes extending through the metal layer 120. In an exemplary embodiment, the metal layer 120 is a layer of palladium with a thickness of approximately 20 nm.

With continued reference to FIG. 2, the metal layer 120 may be formed using typical metal deposition methods including, but not limited to, sputtering and evaporation. In some embodiments, a seed layer (not shown) made of, for example, titanium, may be deposited between the metal layer 120 and the substrate 110 to improve adhesion of the metal layer 120 to the substrate 110. In some embodiments, the seed layer may have a thickness of approximately 1 nm.

Referring to FIG. 3, a metal structure 210 may be formed from using the metal layer 120. In an exemplary embodiment, the metal structure 210 may be formed using a typical masking and etching process, such as defining the boundaries of the metal structure 210 using a photolithographic layer and then using a RIE process to remove unwanted material from the metal layer 120. In an exemplary embodiment where the metal layer 120 is made of palladium, the RIE process may use argon as an etchant to remove material from the metal layer 120. In some embodiments, a hard mask layer, such as a bi-layer of tantalum and silicon oxide, may be used to prevent overetching of the metal layer 120. In such embodiments, any hard mask layer may be removed after forming the metal structure 210.

With continued reference to FIG. 3, the metal structure may include a first end 212 a, a second end 212 b, and an intermediate portion 214 between, and joined to, the first end 212 a and the second end 212 b. In some embodiments, including the embodiment depicted in FIG. 3, the intermediate portion 214 may be a nanostructure such as a nanowire. In embodiments where the intermediate portion 214 is a nanowire, the intermediate portion 214 may have a width x, where x is less than approximately 100 nm. The minimum width x of the intermediate portion 214 may be determined by the method used to form the metal structure 210. Typically, the width x may range from approximately 15 nm to approximately 20 nm.

In other embodiments, the intermediate portion 214 may include metal regions of other shapes and sizes. For example, intermediate portion 214 may include a nanowire having a fan-out structure, where the ends of the nanowire are wider than the middle of the nanowire to reduce contact resistance. In further examples, the intermediate portion 214 may be a metal region of the same width as the first end 212 a and the second end 212 b, or may be multiple nanowires or fins spanning from the first end 212 a to the second end 212 b.

In other embodiments, the metal structure 210 may be a single metal layer not including a first end or a second end. In such embodiments, the entire metal structure 212 may be made porous by the etching process described below in conjunction with FIGS. 4-7, excluding the step of forming protective layers over portions of the metal structure 210.

Referring to FIG. 4, a masking layer 310 may be formed so that the masking layer 310 covers the metal structure 210. The masking layer 310 may be made of any material which may be etched selectively relative to the material of the metal structure 210. In embodiments where the metal structure 210 is made of palladium, the masking layer 310 may be made of a nitride, such as titanium nitride, silicon nitride, or tantalum nitride, or an oxide such as silicon oxide.

With continued reference to FIG. 4, the masking layer 310 may have a thickness sufficient to just cover the metal structure 210. In an exemplary embodiment, the masking layer 310 may have a thickness z above the top surface of the metal structure 210. In an exemplary embodiment, z may be less than 5 nm, though greater thicknesses are explicitly contemplated.

Referring to FIG. 5, protective layers 410 may be formed above the masking 310 over the first end 212 a and the second end 212 b of the metal structure 210. The protective layers 410 may be made of any suitable material capable of withstanding the etching process described below in conjunction with FIG. 6, so that the first end 212 a and the second end 212 b are protected while etching the intermediate portion 214. In an exemplary embodiment, the protective layers 410 may be made of a photoresist material.

In some embodiments, the protective layers 410 may include a hard mask layer, such as silicon oxide or silicon nitride, in addition to, or in lieu of, the photoresist material, to increase the durability of the protective layers 410. The material of the protective layers 410 may be selected so that the protective layers 410 may later be selectively removed without removing any of the metal structure 210.

Referring to FIG. 6, the masking layer 310 may be etched to expose the intermediate portion 214 of the metal structure 210. The masking layer 310 may be etched with a reactive ion etching process using an etchant gas including a fluorocarbon mask etchant and a metal etchant. In embodiments where the masking layer 310 is made of titanium nitride and the metal structure 210 is made of palladium, the fluorocarbon mask etchant may include chlorine (Cl₂) and fluoroform (CHF₃) and the metal etchant may include argon (Ar). Alternative dry metal etchants for palladium may include methane (CH₄). Etchants for other masking layer materials and metal structure materials are known in the art. The fluorocarbon mask etchant may cause non-uniform etching of the masking layer 310 by fluoride carbon re-deposition. While the mask etchant etches away the masking layer 310, the metal etchant may sputter away portions of the metal structure 210 and transfer the non-uniform etch pattern of the masking layer 310 to the metal structure 210. This uneven etching may result in the formation of a plurality of pores 510 within the intermediate portion 214.

In an exemplary embodiment, the etch process may occur in two stages. In the first stage, the reactive ion etching process chamber may be set at approximately 1000 Watts (W) source power, approximately 250 Watts (W) bias power, and approximately 8 millitorr (mT) pressure and have etchant flow rates of approximately 100 standard cubic centimeters per minute (sccm) Ar, 35 sccm Cl₂, and 10 sccm CHF₃. In the second stage, the reactive ion etching process chamber may be set at approximately 500 W source power, approximately 20 W bias power, and approximately 8 millitorr (mT) pressure and have etchant flow rates of approximately 100 standard cubic centimeters per minute (sccm) Ar and 35 sccm Cl₂. In embodiments where a two step process is used, the amount of the mask etchant may be reduced, so that the rate of the metal sputtering is increased relative to the etch rate of the masking layer 310.

Referring to FIG. 7, the protective layers 410 and any remaining portions of the masking layer 310 may be removed to expose the metal structure 210. In some embodiments, the remaining portions of the masking layer 310 may be left in place. In such embodiments, the remaining portions of the masking layer 310 may serve as contact pads for probing the electrical current through the intermediate portion 214 of the metal structure 210.

The protective layers 410 and any remaining portions of the masking layer 310 may be removed using typical etch processes, either wet or dry, that are suitable for the specific materials used. For example, where the masking layer 310 is made of titanium nitride, the remaining portions of the masking layer 310 may be removed using a wet etch of hydrogen chloride and hydrogen peroxide, or a wet etch of ammonium hydroxide and hydrogen peroxide.

With continued reference to FIG. 7, after removing protective layers 410 and the remaining portions of the masking layer 310, the metal structure 210 remains on the substrate 110. The intermediate portion 214 of the metal structure 210 is perforated by the plurality of pores 510, while the first end 212 a and the second end 212 b are not porous. The plurality of pores 510 may not be uniform in size. The plurality of pores 510 may also be randomly distributed across the intermediate portion 214. Depending on the specific etch conditions, the plurality of pores 510 may have a maximum diameter of approximately 10 nm, though greater diameters are explicitly contemplated. In some embodiments, the average diameter of the plurality of pores 510 may be approximately 5 nm. The plurality of pores 510 may extend fully through the metal structure 210 in the intermediate portion 214. In other embodiments, at least some of the plurality of pores 510 may extend only partially into the metal structure 210, so that the intermediate portion 214 is only pitted rather than being fully porous.

Once the protective layers 410 and the remaining portions of the masking layer 310 are removed as desired, the structure may continue to undergo typical microelectronic fabrication processes to form desired structures such as chemical sensors. Where the metal structure 210 is used to form part of a chemical sensor, such fabrication processes may include forming electrical contacts to the first end 212 a and the second end 212 b, so that the resistance of the intermediate portion 214 may be measured. Because the method of forming the porous metal structure 210 described above uses typical semiconductor fabrication processes such as metal deposition, photolithography, and reactive ion etching, it may be more easily incorporated into a microelectronic process flow than methods of forming nanostructures that require a bottom-up approach, such as solution-phase reactions or surface dispersion.

The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiment, the practical application or technical improvement over technologies found in the marketplace, or to enable other of ordinary skill in the art to understand the embodiments disclosed herein. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated but fall within the scope of the appended claims. 

What is claimed is:
 1. A method of perforating a metal structure, the method comprising: forming a masking layer above the metal structure; and removing at least a portion of the masking layer using a reactive ion etching process with an etching gas containing a fluorocarbon mask etchant, and a metal etchant, wherein removing the masking layer forms a plurality of pores in the metal structure, wherein the masking layer comprises a material selected from the group consisting of titanium nitride, silicon nitride, tantalum nitride, and silicon oxide.
 2. The method of claim 1, wherein the metal structure comprises a metal selected from the group consisting of palladium, gold, titanium, and platinum.
 3. The method of claim 1, wherein the mask etchant comprises chlorine and fluoroform.
 4. The method of claim 1, wherein the metal etchant comprises argon or methane.
 5. The method of claim 1, wherein the plurality of pores have an average diameter of less than approximately 10 nm.
 6. A method of forming a porous metal structure, the method comprising: forming a metal structure above a substrate, wherein the metal structure comprises a first end region, a second end region, and an intermediate region between the first end region and the second end region; forming a masking layer above the metal structure, wherein the masking layer covers the metal structure; forming a protective layer above the first end region and the second end region; and etching the masking layer away from above the intermediate region using a reactive ion etching process with a mask etchant and a metal etchant, wherein etching the masking layer forms a plurality of pores in the intermediate region.
 7. The method of claim 6, wherein the metal structure comprises a metal selected from the group consisting of palladium, gold, titanium, and platinum.
 8. The method of claim 6, wherein the intermediate region comprises a nanowire.
 9. The method of claim 6, wherein the masking layer comprises a material selected from the group consisting of titanium nitride, silicon nitride, tantalum nitride, and silicon oxide.
 10. The method of claim 6, wherein the protective layer comprises a material selected from the group consisting of photoresists, oxides, and nitrides.
 11. The method of claim 6, wherein the mask etchant comprises chlorine and fluoroform.
 12. The method of claim 6, wherein the metal etchant comprises argon or methane.
 13. The method of claim 6, wherein the plurality of pores have an average diameter of less than 10 nm. 